Typical process control methodology involves specifying control limits for a physical parameter of unit products produced by the manufacturing process. To specify control limits, a parameter, such as weight, volume, moisture content, and the like is monitored over a specified period of time. Once a sufficient amount of information is collected, statistical analysis is applied to the product parameter to determine statistical values such as the need, standard deviation, range, and the like. Once the statistical values are determined, upper and lower process control limits are specified for the product parameter.
FIG. 1 shows a typical process control chart in which a product parameter, such as weight, is shown by line 10. The weight varies about a mean value denoted by the centerline 12. Based on the variation of the product weight over time, a lower process control limit 14 and an upper process control limit 16 are determined using statistical analysis. The upper and lower control limits are further adjusted based on the particular requirements of the manufactured product. For example, the buyer of the unit product may require that the product weight fall within a specified range. Other factors, such as weight requirements imposed by governmental regulations can also dictate values for the upper or lower control limit. Accordingly, the upper and lower control limits 14 and 16 can be further adjusted to trigger an alert if the unit product weight 10 goes above or below the upper or lower control limits.
In order to avoid producing unit products that fall below a specified weight, manufacturers tend to bias the process control parameters toward the upper end of the weight range. In this way, undesired low-weight products emerging from the manufacturing process are minimized. While biasing the control limits toward the upper end of the weight range can help ensure that underweight unit products will be minimized, the possibility of producing overweight unit products is increased. By producing unit products in excess of an upper weight limit, excess manufacturing costs are realized. While the buyer of the unit products can benefit from overweight products, the manufacturer often charges a sales price based on a predetermined specified weight. Accordingly, the manufacturer can suffer a monetary loss on every overweight unit product produced by the manufacturing process. In some cases, where overweight unit products are produced, the excess must be discarded in situations where the packaging of the unit products does not permit a product weight or volume beyond an upper limit. In such a situation, the excess weight of each unit products must be discarded or recycled and either represents a monetary loss or reduces the overall efficiency of the manufacturing process, or both.
Thus, there is a need for improved process control methods that increase the efficiency of the manufacturing process by reducing the amount of excess product, while maintaining a physical or chemical characteristic of the unit products within a desired control range.